The present invention relates generally to the public switched telecommunication network and to the common channel signaling system that carries the messages that control the routing of toll and other telecommunication system connections. The call-routing messages are digital in nature and can be transmitted in any number of different digital transmission protocols. More particularly, the present invention relates to the organization of each signal transfer point of a common channel signaling system of the public switched telecommunication network and the addressing of each such geographically-distributed signal transfer point which serves a plurality of geographically-separated public switched telecommunication network switches.
The object of public telecommunication switchesxe2x80x94sometimes called central office switches, toll switches, or service switching points (SSPs)xe2x80x94and the common channel signaling system (CCS) that controls the call-serving interconnections between SSPs is to connect, establish, or set up a telecommunication connection, most easily referred to as a xe2x80x9cvoice path,xe2x80x9d through as many SSPs and trunks (that interconnect SSPs) as necessary. That voice path then extends from a telecommunication customer""s calling telephone or other terminal equipment (calling party) to a called telephone or other terminal equipment (called party), which might be located across town or across the country from the calling party. To establish such a voice path, the SSPs must be instructed as to the identification of the called party and be instructed to interconnect the calling party and the called party through a series of interconnecting SSPs and trunks of the public telecommunication network.
Formerly, the inter-SSP signaling necessary to establish such a communication or voice path used a series of dial-like tones. Such a series of signaling tones to set up such a path was sent over the same communication or voice path or inter-SSP trunk channel that would later carry the actual communication for the terminal-to-terminal conversation or communication. These tones were often audible to the caller.
Using the same channel for setting up or establishing a call path and for the later conversationxe2x80x94sometimes called voice-path signalingxe2x80x94had at least two disadvantages. The time of the trunk and other facilities during call set-up could not be billed to the customer and was thus partly wasted. Also, having the voice path carry path-control tones that were audible to the customer sometimes led unscrupulous customers, often at pay phones, to cut in with their own tone generators and misuse the public telecommunications network.
In order to enable the telecommunication service providers better to control the operation and operating costs of their networks, the call set-up signals were removed from the voice path and sent along separate, digital signaling channels. A great many call set-up signals for a great many calls can be sent very quickly over the same, common digital communication channel. Therefore, the system of sending digital call-set-up signals over a common signaling path was called common-channel-signaling (CCS).
A lot of common channel signaling messages can be sent over one channel. However, a switching service point (SSP) can easily have so much toll traffic that it needs several CCS channels to both send and receive CCS traffic. Also, each CCS message must be routed properly and may require different routing from almost every other CCS message at any given SSP. Therefore, the CCS messaging system must employ a small digital switching network of its own, in parallel with and almost mirroring the switches and trunks of the public switched telecommunication network.
The common channel signaling (CCS) digital switching networkxe2x80x94each switch being called a signal transfer point (STP)xe2x80x94usually has several digital interface circuits and a digital switch sometimes resident or collocated at a SSP switching office. The CCS digital interface circuits and digital switch (together constituting the STP) are usually administered at that office, along with the SSP. For redundancy, every SSP must be served by at least two STPs, at least one of which must not be collocated with the SSP. In addition, redundancy requirements dictate that each STP must be able to communicate with at least three other STPs.
Each CCS signal transfer point (STP) has its own digital address (called a point code or xe2x80x9cPCxe2x80x9d) within the CCS network, much as a company, a factory, or an engineering department might have its own e-mail domain address on the internet Essentially, with most central or toll switching offices (SSPs) having their own point codes or PCs within the CCS network as well as a separate PC for each STP, addressing and routing of CCS messages can be a needlessly complex problem. This can result in a necessary but undesirable proliferation of STPs and their PCs. Essentially, with most central or toll switching offices (SSPs) having their own point codes or PCs within the CCS network as well as a separate PCs for each STP[s], addressing and routing of CCS messages can be a needlessly complex problem. This can result in a necessary but undesirable proliferation of STPs and their PCs.
Also, the digital CCS interconnections between SSPs and the STPs that serve them as well as the interconnections between STPs typically use a digital transmission protocol known SS7. The SS7 protocol, as with most digital transmission protocols, has a number of bytes reserved for the sending address, a number of bytes reserved for the receiving address and a portion for the data or message. With a proliferation of SSPs, STPs, and other portions of the telecommunication infrastructure, the portion of the SS7 protocol reserved for the sending and receiving addresses is expected, eventually, to become limiting.
The SS7 protocol messages are normally communicated between the SSPs and the STPS that serve them and also between the STPs themselves at a standard 56 or 64 kilobits per second (KB). However, with new high-speed links, the SS7 signals could be communicated at 1.544 megabits per second (MB). These interconnections are rather expensive for their normal transmission speed capability and are usually kept short. Also, when the amount of CCS traffic on an SS7 link gets too high, that SS7 link must be paralleled by a second SS7 link. Since each STP must be connected to three STPs and each of those three other STPs must be able to take the highest traffic volume from that STP, several of the SS7 links that interconnect those three STPs to each other and to still other STPs must also be paralleled, which multiple links are referred to as Linksets.
Whenever another switching office or switching service point (SSP) is added to the public switched telecommunications network or when other changes are made, such as changing direct distance dialing area code numbers, as has been happening with increasing frequency in recent years, the many STPs and the databases that they use must be updated to reflect the necessary expansion of the public switched telecommunications network. With the large numbers of STPs extant, in order to be safely separated from their SSPs and yet have short SS7 links, the capitol investment in the common channel signaling (CCS) system is rising fast.
Accordingly, the present invention is directed to maximizing the efficiency of the common channel signaling (CCS) system of the public switched telecommunication network by distributing the constituent parts of each CCS signal transfer point (STP) so as to allow a STP to serve a larger number of geographically-dispersed of SSPs with each served SSP having a resident or non-collocated but nearby portion of a distributed STP. The SS7 communication links between the SSP and the portion of a STP can then be kept very short. The resultant necessary communication between the geographically-distributed constituent parts of a STP is then accomplished at a much higher speed with any suitable digital transmission protocol. In this way, the capitol costs of the STP hardware are minimized and the length of 56 KB SS7 links are also shortened, thereby further reducing costs. Also, by minimizing the number of STPs contained in the CCS network the number of STP addresses necessary is also minimized.
It is also an object of the present invention more fully to utilize the capability of each CCS STP by causing each STP to be distributed over a large geographic area in order to serve a large number of widely disbursed SSPs without extending costly communication channels over long distances.
Additional objects and advantages of the present invention are set forth in part in the description which follows and in part will be apparent from the following detailed description or may be learned by practice of the invention. Objects and advantages of the invention will be realized from the elements and combinations particularly pointed out in the appended claims.